Person-hours of actual use; app purchase & use; web use; user satisfaction.

For example, if you're developing a business website, iOS dominates the market, because optimizing for Android is nigh impossible, and iOS users have more money, and spend more money (on average, your market segment may differ, mine does).

Or, if you're considering making an app, iOS dominates the market, because Android users don't like using apps, much less paying for them.